The present invention provides a novel method and apparatus for discharging a cake of solids formed on an electrode structure. The present invention uses a blowback means for discharging the cake of suspended solid, the blowback means consisting essentially of expansion of the membrane wall by injection of gas or liquid into the electrode structure, and/or removing of vacuum means from the electrode structure. The electrode structure according to the present invention is primarily used in an electrically augmented vacuum filter.
The use of an electrically augmented vacuum filter (hereinafter referred to as EAVF) system for dewatering a suspension of solids in a carrier liquid is described in U.S. Pat. Nos. 4,168,222 and 4,207,158. These patents describe a means for dewatering a suspension of solids in an electric field which is controllably maintained between opposing electrodes, to cause the solids to migrate relative to the carrier liquid to form a layer or cake on one of the electrode structures in which the electrode element is positioned within ion-pervious walls and immersed in a selected electrolyte, while allowing carrier liquid to be withdrawn under vacuum in the opposite direction through the liquid-pervious wall of a hollow, counter electrode structure, and wherein the layer or cake material may be detached from the first electrode structure during exposure from the suspension.
Typically, a vacuum is applied internally to the electrode structure to reduce the internal pressure on the ion pervious wall, such that when the electrode is in the raised position for doctoring of the cake of solids formed thereon, the vacuum prevents the loss of electrolyte and/or rupture of the filter media. The aforementioned patents particularly provide for the use of doctor blades 17 and 18 such that a pair of symmetrically disposed doctor blades swingable about their horizontal axes as between neutral position and cake stripping position. The cake material thus being stripped may be carried away by conveyor means provided therein.
Repetitive scraping of the doctor blade directly on the ion pervious wall may damage the wall surface by abrading, wearing, tearing or ripping. This is especially true when the doctor blade tips have been worn or abraded by hard dry cakes, such as kaolin clay. To prevent engagement of the doctor blades with the electrode ion pervious wall, the blades are normally positioned to leave a cake heel, i.e. 1/4 to 1/2 inch layer of cake on the electrode structure. While this provides for long life of the ion pervious wall, there are some other consequences which negate the benefits. For instance, where the adhesion of the cake to the ion pervious wall is low, the cake heel may subsequently and uncontrollably disengage from the anode, fall into the bath and interfere with the process.
Another problem could occur if the deposited cake is composed of heat sensitive materials. Heat is generated in the cake due to the electric current passing thru it. If the cake is not removed entirely between cycles, heat will build up in the heel until radiation and convective losses equal the generation term. In the case of polymer latex resins, for instance, the temperature of the heel could, after repetitive cycles, exceed the glass transition temperature of the polymer, causing the latex particles to fuse together resulting in an electrically impervious cake heel which catastrophically stops the electrofiltration process.
In yet another case, where the selection of ion pervious wall material results in a cake which is dialyzed over repetitive doctoring cycles. This would result in excessive power losses in the heel, again causing high heat generation, but also, when the conductivity drops low enough, water will split into acid and hydroxide, resulting in an acid flocculated cake.
Thus, it is preferable in some instances to provide means for discharging substantially all of the cake formed on the membrane walls of the electrode structure. It is also preferable for economic reasons to provide a discharge means which does not rip or tear the membrane wall. The present invention provides a method and apparatus which overcomes the aforementioned disadvantages. The present invention also includes many additional advantages which will be further described below.